Web caching is the caching of web documents (e.g., HTML pages, images) to reduce bandwidth usage, server load, and perceived lag. A web cache stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met.[1]
It should not to be confused with a web archive, a site that keeps old versions of web pages.
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Controlling Web caches
HTTP defines three basic mechanisms for controlling caches: freshness, validation and invalidation.
- Freshness allows a response to be used without re-checking it on the origin server, and can be controlled by both the server and the client. For example, the Expires response header gives a date when the document becomes stale, and the Cache-Control: max-age directive tells the cache how many seconds the response is fresh for.
- Validation can be used to check whether a cached response is still good after it becomes stale. For example, if the response has a Last-Modified header, a cache can make a conditional request using the If-Modified-Since header to see if it has changed.
- Invalidation is usually a side effect of another request that passes through the cache. For example, if URL associated with a cached response subsequently gets a POST, PUT or DELETE request, the cached response will be invalidated.
Legal Issues
In 1998 the DMCA added rules to the United States Code (17 U.S.C. § 512) that relieves system operators from copyright liability for the purposes of caching.
Web cache software
- ApplianSys CACHEbox (appliance)
- Blue Coat ProxySG (appliance)
- Squid (software)
- WinGate (software)
- [CCC]
See also
References
- ^ Geoff Huston. "Web Caching". Cisco. The Internet Protocol Journal - Volume 2, No. 3. http://www.cisco.com/web/about/ac123/ac147/ac174/ac199/about_cisco_ipj_archive_article09186a00800c8903.html. Retrieved 2009-09-10.
- Ari Luotonen, Web Proxy Servers (Prentice Hall, 1997) ISBN 0-13-680612-0
- Duane Wessels, Web Caching (O'Reilly and Associates, 2001). ISBN 1-56592-536-X
- Michael Rabinovich and Oliver Spatschak, Web Caching and Replication (Addison Wesley, 2001). ISBN 0-201-61570-3
External links
- Is the Internet Heading for a Cache Crunch?
- Caching Tutorial for Web Authors and Webmasters
- Web Caching and Content Delivery Resources
- Web Caching, Web caching in general with some references to SQUID
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